Precursors as enablers of ALD technology: Contributions from University of Helsinki

The electrical characteristics of SrTiO3/Al2O3 (160 nm up/90 nm down) laminated film capacitors using the sol-gel process have been investigated. SrTiO3 is a promising and extensively studied high-K dielectric material, but its leakage current property is poor. SrTiO3/Al2O3 laminated films can effectively suppress the demerits of pure SrTiO3 films under low electric field, but the leakage current value reaches to 0.1 A/cm2 at higher electric field (>160 MV/m). In this study, a new approach was applied to reduce the leakage current and improve the dielectric strength of SrTiO3/Al2O3 laminated films. Compared to laminated films with Au top electrodes, dielectric strength of laminated films with Al top electrodes improves from 205 MV/m to 322 MV/m, simultaneously the leakage current maintains the same order of magnitude (10−4 A/cm2) until the breakdown occurs. The above electrical characteristics are attributed to the anodic oxidation reaction in origin, which can repair the defects of laminated films at hig...

Electrical characteristics of SrTiO3/Al2O3 laminated film capacitors

The resistive switching (RS) properties of strontium titanate (Sr1+xTi1+yO3+(x+2y), STO) based metal-oxide-metal structures prepared from industrial compatible processes have been investigated focusing on the effects of composition, microstructure, and device size. Metastable perovskite STO films were prepared on Pt-coated Si substrates utilizing plasma-assisted atomic layer deposition (ALD) from cyclopentadienyl-based metal precursors and oxygen plasma at 350 °C, and a subsequent annealing at 600 °C in nitrogen. Films of 15 nm and 12 nm thickness with three different compositions [Sr]/([Sr] + [Ti]) of 0.57 (Sr-rich STO), 0.50 (stoichiometric STO), and 0.46 (Ti-rich STO) were integrated into Pt/STO/TiN crossbar structures with sizes ranging from 100 μm2 to 0.01 μm2. Nano-structural characterizations revealed a clear effect of the composition of the as-deposited STO films on their crystallization behavior and thus on the final microstructures. Local current maps obtained by local-conductivity atomic force microscopy were in good agreement with local changes of the films' microstructures. Correspondingly, also the initial leakage currents of the Pt/STO/TiN devices were affected by the STO compositions and by the films' microstructures. An electroforming process set the Pt/STO/TiN devices into the ON-state, while the forming voltage decreased with increasing initial leakage current. After a RESET process under opposite voltage has been performed, the Pt/STO/TiN devices showed a stable bipolar RS behavior with non-linear current-voltage characteristics for the high (HRS) and the low (LRS) resistance states. The obtained switching polarity and nearly area independent LRS values agree with a filamentary character of the RS behavior according to the valence change mechanism. The devices of 0.01 μm2 size with a 12 nm polycrystalline stoichiometric STO film were switched at a current compliance of 50 μA with voltages of about ±1.0 V between resistance states of about 40 kΩ (LRS) and 1 MΩ (HRS). After identification of the influences of the films' microstructures, i.e., grain boundaries and small cracks, the remaining RS properties could be ascribed to the effect of the [Sr]/([Sr] + [Ti]) composition of the ALD STO thin films.

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Impact of composition and crystallization behavior of atomic layer deposited strontium titanate films on the resistive switching of Pt/STO/TiN devices

Strontium titanate, SrxTiyOz (STO), thin films with various cation stoichiometries were deposited by plasma-assisted atomic layer deposition (ALD) using cyclopentadienyl-based metal precursors and oxygen plasma as counter-reactant. [Sr]/([Sr] + [Ti]) compositions ranging from 0.46 to 0.57 were obtained by changing the (SrO)/(TiO2) ALD cycle ratios. As-deposited 15–30 nm thick SrxTiyOz films prepared at 350 °C on Pt-coated silicon substrates were amorphous. Post-annealing at 600/650 °C for 10 min under N2 gas resulted in a crystallization into the perovskite phase. Stoichiometric STO and Sr-rich STO films exhibited a certain degree of (111) texture while the Ti-rich STO films showed a lower degree of crystallization. Crystallized layers exhibited a smaller band gap Eg than amorphous ones, while within the stoichiometry series the value of Eg increased with increasing Sr-content. Within the stoichiometry series Pt/STO/Pt structures with Sr-rich STO films showed the lowest leakage current densities. At 1.0 V values of about 2 × 10−8 and 5 × 10−6 A cm−2 were obtained for the as-deposited and the annealed films, respectively. Highest capacitance density was obtained for 15 nm polycrystalline stoichiometric SrTiO3 films resulting in a capacitor equivalent thickness CET of about 0.7 nm. Pt/SrxTiyOz/Pt capacitors with the STO being in amorphous state exhibited a positive voltage nonlinearity factor α of about 400 ppm V−2, while the negative α-values for crystallized films showed a systematic variation with the stoichiometry, the degree of crystallization and the thickness of the STO layer. This demonstrates that a broad performance range of MIM capacitors is accessible by controlling the stoichiometry and the degree of crystallization of plasma-assisted ALD SrxTiyOz thin films.

Influence of stochiometry on the performance of MIM capacitors from plasma-assisted ALD SrxTiyOz films

Effect of Al2O3 insertion on the electrical properties of SrTiO3 thin films: A comparison between Al2O3-doped SrTiO3 and SrTiO3/Al2O3/SrTiO3 sandwich structure

The possibilities to grow thin films of SrTiO3 and Al2O3 by atomic layer deposition for stacked metal–insulator–metal capacitors have been investigated in this work. In order to tune the functional properties of the capacitors, different processing steps have been employed to realize different combinations of the dielectric stacks. Electrical properties, extracted after the postdeposition annealing and sputter deposition of the Au top electrodes, indicated that the metal–insulator–metal (MIM) structures with additional Al2O3 layer provided better leakage currents densities, compared to the ones with single SrTiO3 based MIM capacitors, but the dielectric constant values have also decreased if additional Al2O3 film was inserted. Attempts to optimize the properties of the MIM stacks have been done by manufacturing heterostructures of Al2O3/SrTiO3/Al2O3 as well as SrTiO3/Al2O3/SrTiO3. In the first case, Al2O3 prevented the crystallization of SrTiO3 in the multilayer dielectric structure and therefore reduced the total capacitance density of the particular MIM stack, whereas the SrTiO3/Al2O3/SrTiO3 stack was found to possess superior electrical properties. Leakage current density as low as ∼10−8 A/cm2 at 2 V and the dielectric constant value of 40 have been extracted.

Tom Blomberg

Papers

Properties of stacked SrTiO3/Al2O3 metal–insulator–metal capacitors

(Invited) ALD and AVD Grown Perovskite-type Dielectrics for Metal-Insulator-Metal Applications